My understanding is that it has nothing to do with whether he's doing it for profit or fun. It has to do with you shipping a gun to a non-licensed person. I used to have a manufacturer's FFL and now have a C&R FFL.

The laws governing this were put in place, not to prevent someone from making a stock for you, but to prevent a person from recieving a firearm that is not entitled to recieve it. Gunsmiths are required to obtain a FFL to conduct their trade regardless of whether they charge for their service or do it for free.